The present invention relates to a novel method in connection with a fault tolerant engine control system, wherein a combustion feedback signal is derived by measuring, in a combustion chamber, one or more combustion related parameters during a chosen time period of a first combustion cycle. For the corresponding time period, an ideal reference signal for said parameters has been previously determined. The method is primarily intended for control of Otto engines or Diesel engines.
In engine control, it is a challenge to keep engine efficiency and combustion stability as high as possible while minimising emissions.
Control of Otto engines basically amounts to controlling three primary variables: ignition timing and fuel and air injected into the cylinder. For the two latter both the mass and the timing are important and these are controlled separately using different actuators such as the throttle, the fuel injectors, and the intake valves depending on engine design and mode of operation. For Diesel engines the main control variables are timing and mass of injected fuel. The main actuators for diesel engine control are, consequently, the fuel injectors. In today""s engine control systems, most of the control functionality is implemented in form of look-up tables, which give the optimal ignition timing, say, for a certain operating point of the engine and at certain prevailing ambient condition. These systems require extensive calibration tests to meet the performance requirements under all driving conditions, including varying speed and load, fuel quality, air temperature, air pressure, air humidity, etc. Calibration of an engine management system is therefore typically a very time consuming and expensive task and accordingly there is a need for other control possibilities, especially since the requirements are continuously augmented.
It has been suggested to use continuous measurements of combustion conditions (combustion feedback signal) in order to eliminate the need of extensive calibration. However, known systems which use continuous measurements for engine control all show some drawbacks, as will be explained later. Ionisation current measurements and in-cylinder pressure measurements are two possible ways of obtaining desired information (combustion feedback signal) for engine control, as is known from e.g. SE-504197. The combustion feedback signal can be measured either directly in the combustion chamber, (as is known per se from e.g. R. Mxc3xcller, M. Hart, A. Truscott, A. Noble, G. Krxc3x6tz, M Eickhoff, C. Cavalloni, and M. Gnielka, xe2x80x9cCombustion Pressure Based Engine Management Systemxe2x80x9d, SAE paper no. 2000-01-0928, 2000; J. Auzins, H. Johansson, and J Nytomt, xe2x80x9cIon-gap sense in misfire detection, knock and engine controlxe2x80x9d, SAE paper no. 950004, 1995) or indirectly using non-intrusive sensors (as is known per se from, e.g. M. Schmidt, F. Kimmich, H. Straky, and R. Isermann, xe2x80x9cCombustion Supervision by Evaluating the Crankshaft Speed and Accelerationxe2x80x9d, SAE paper no. 2000-01-0558, 2000; M. Sellnau, F. Matekunas, P. Battiston, C.-F. Chang, and D. Lancaster, xe2x80x9cCylinder-Pressure-Based Engine Control Using Pressure-Ratio-Management and Low-Cost Non-Intrusive Cylinder Pressure Sensorsxe2x80x9d, SAE paper no. 2000-01-0932, 2000). As described in said publications (and publications defined below) these measurements are used for closed-loop engine control and that primary benefits of such closed-loop engine control are lower fuel consumption and emissions. Secondary benefits are various possible improvements in terms of improved misfire and knock detection, individual cylinder air/fuel ratio control, camshaft phasing, control of start-of-combustion, EGR rate control, etc. See e.g. Muller et al. (2000); Sellnau et al. (2000) according to the above, or H. Wilstermann, A. Greiner, P Hohner, R. Kemmler, R. Maly, and J. Schenk, xe2x80x9cIgnition System Integrated AC Ion Current Sensing for Robust and Reliable Online Engine Controlxe2x80x9d, SAE paper no. 2000-01-0553, 2000; or L. Nielsen and L. Eriksson, xe2x80x9cAn Ion-Sense Engine Fine-Tunerxe2x80x9d, IEEE Control Systems, 1998.
However, these known engine control systems all have in common that they are not fault tolerant, i.e. it may control/change the wrong variable since the interrelation between the different variables may be very complex and therefore extremely difficult to handle in both open-loop and closed-loop control systems. If for instance the fuel/air mixture is not optimised this may lead to a changed burn rate which in turn leads to a change in the peak pressure position that is used for closed-loop ignition timing control (e.g. SE 504 197). This leads to a suboptimisation of the engine control, which results in decreased efficiency of the engine and higher emission levels. There is also a risk that a multiple loop control system may cause drastic interference problems. Moreover they require time consuming tuning. The invention alleviates all these problems.
It is an object of the present invention to present a fault tolerant engine control system by utilising combustion feedback information, which is achieved by a method according to the invention, as presented in claim 1.
The proposed system leads to improved performance and increased functionality compared to existing solutions for engine control, and is conceptually simpler and therefore also more cost efficient.
According to a further aspect of the invention model-based diagnosis is used. By using model-based diagnosis, preferably parametric, a unique highly efficient diagnosis system can be designed and quantitative information about the size of the fault be obtained, which enables efficient adaptation of the control law, for optimisation of the performance of an engine.
Parametric modelling of the ionisation current signal is indeed known per see from, e.g. SE 504 197, which suggests determining the time location of the pressure peak during a combustion cycle, by detecting the ionisation degree in the combustion chamber and fitting a parametric model to the measured ionisation current. A peak point in the model curve is used in order to determine the time location of the pressure peak during the combustion cycle. According to a preferred embodiment, the measured ionisation current is parameterised by being fitted to two consecutive and partially overlapping Gaussian functions. It is also suggested to control the ignition timing of the combustion cycle by controlling the pressure peak to lie within a predetermined time interval, the location of which depends at least on the prevailing load and motor speed. In H. Klxc3x6vmark. xe2x80x9cEstimating Air/Fuel Ratio from Gaussian Parameterisations of the Ionisation Currents in Internal Combustion SI Enginesxe2x80x9d, Master thesis EX 065/1998, Chalmers University of Technology, 1998, it has also been suggested to parameterise the measured ionisation current in order to estimate air/fuel ratio.
However, none of the latter described systems are able of performing fault tolerant engine control and they also only feature single-variable optimisation, not multivariable optimisation.
The proposed engine control system combines three different techniques in a unique manner resulting in a fault tolerant engine control system, which may provide an enormous progress in controlling engine efficiency, high combustion stability and minimised emissions.
According to another aspect of the invention, the type of faults that are detected and which the control system is adapted to handle is one or more of the fault situations in the group that comprises misfire, pre-ignition, knock intensity, wrong location of peak pressure, wrong air-fuel ratio and wrong EGR rate.